Many evolutionary approaches to understanding human cooperation emphasize the importance of inter-group conflict. Recent models predict that if inter-group competition was an enduring selective force over human evolutionary history, it would favour motivations that increase both (i) egalitarianism, to suppress within-group competition, and (ii) parochialism, to favour in-group members.

In two diverse populations, we test this hypothesis using simple economic decision experiments among children and adults who were differentially affected by war. From the Republic of Georgia, our experiments show that older children who were more war-afflicted are more egalitarian towards in-group members, even six months post-conflict. In Sierra Leone, the same experiments reveal that adults who were older children or adolescents during the war are also more egalitarian towards in-group partners, even a decade post-conflict. Meanwhile, those who were younger than age 7 reveal no short-term effects (Georgia) and those older than 20 reveal only much muted long-term effects (Sierra Leone).

These results indicate that experiencing inter-group conflict during the window from middle childhood through adolescence increases parochial egalitarianism. Such motivational shifts have implications for understanding the relationship between warfare, polity building and post-conflict recovery.